The present invention is generally directed to electrical probes used to make temporary electrical connections to contact pads on semiconductor wafers. These probes provide an important electrical interconnection mechanism for applying voltages, for inserting signal inputs, and for measuring signal outputs for semiconductor wafers at stages of manufacture prior to their being cut into individual integrated circuit chips.
Generally speaking, the probes discussed herein include a large plurality of individual electrical contacts. These contacts typically number in the thousands. These contacts are arranged so that they match correspondingly placed electrically conductive pads which may be designed into a semiconductor wafer for test purposes. These probes possess many competing characteristics which make their design and construction very difficult. For example, it is important that the contacts do not experience any significant distortion which would interfere either with their horizontal or with their vertical alignment. This could result in non-alignment with the corresponding contact pads on the wafer. In this regard, it is noted that these pads and the probe""s contacts are both very small and are spaced very close together.
When placed on a wafer, the probe, with its thousands of individual contacts, should make clean electrical contact with the pads on the wafer. Alignment should not only be easy to accomplish for each test, but the behavior and characteristics of the probe (or probe assembly, if you will) should be repeatable over many thousands of tests. With specific reference to the probe of the present invention, as described herein, it is noted that it is designed to withstand up to approximately 250,000 uses. After each test, the individual contact elements should not only return to the same position but should also thereafter still be capable of exerting the same force on the wafer contact pad. Furthermore, in this regard, it is noted that very specific force levels are desired. If the force exerted by the contacts when the probe is placed against a wafer is too small, properly repeatable electrical contact is not made with the wafer pads. On the other hand, the force should not be so large that the contact pads are damaged or affected in any way which would impact future readings obtained through the pad.
It is also very desirable that individual flexible probe contacts move in a vertical direction with substantially no horizontal components to the motion. Clearly, it is desirable that this feature be provided in as simple a manner as possible without the use of complicated mechanical arrangements which have their own mechanisms for high failure rate after only a few thousand uses. The feature of being able to move only in a relatively narrow vertical direction should also be repeatable over many thousands of uses. Additionally, the amount of vertical motion in such probes is typically relatively high in comparison to the dimensions involved. The large extent of this motion contributes to stresses which tend to shorten the useful life of the individual probe contacts.
Thus, it is seen that desirable probes include many thousands of contacts. Each of the contacts moves in a narrowly defined vertical direction. After repeated uses, the probe contacts exert the same force and return to the same positions ready again for the same vertical motion and ready again to exert the same force and make the same quality of physical and electrical contact. With each use the electrical contact resistance remains substantially the same with little or no wear on the probe contact tip. After repeated use, desirable probe contacts remain in substantially the same position as originally designed, both with respect to their horizontal position in an x-y plane and with respect to their vertical position, while still retaining their ability to move through the same vertical displacement with no substantial horizontal component to the motion.
In a preferred embodiment of the present invention an electrical probe assembly comprises a first substantially flat support or base upon which a plurality of flexible, self-supporting wire probes are affixed. The geometry of these wire probes and the materials employed in them constitute an important aspect of the present invention. A substantially flat mask having a plurality of apertures is disposed in a spaced apart relationship with respect to the base. The wire probes extend from the base through apertures in the mask. The wire probes include two oppositely directed bends which provide flexibility to the wire probes. The individual wire probes are plated to a thickness, p, and possess an inner core diameter of d. The two bends in the wire probes produce a horizontal displacement, L, between the point at which the wire probe is affixed to the base and the point at which the wire probe extends through an aperture in the mask. The ends of the wire probes extending through the apertures possess pointed tips for electrical contact with corresponding pads on semiconductor wafers. The parameters L, p, and d cannot be chosen independently while still preserving desirable properties of vertical motion, self-alignment, and long life. These parameters are chosen conjointly so as to define a point within three dimensional space as shown in the figures herein.
Accordingly, it is an object of the present invention to provide an improved probe for use in the testing of semiconductor wafers and similar devices.
It is also an object of the present invention to improve the testing of semiconductor wafers through the use of a more reliable probe used in making contact with pads on these wafers.
It is yet another object of the present invention to construct a probe which exhibits a long useful life even after thousands of repeated uses.
It is a further object of the present invention to construct improved wire probe elements for use in wafer probe assemblies.
It is a still further object of the present invention to construct wire probes which are capable of self-alignment.
It is also an object of the present invention to construct wire probes which are capable of motion in a vertical direction without substantial motion in a horizontal direction.
It is yet another object of the present invention to construct wire probes which provide substantially the same contact force even after thousands of uses.
It is a further object of the present invention to construct probe assemblies that are consistent with current measuring techniques and practices and which would not require retooling operations.
It is a still further object of the present invention to provide improved mechanisms for applying voltages to, injecting signals into, and acquiring output signals from semiconductor wafers and similar devices for test purposes.
It is yet another object of the present invention to construct a probe assembly, especially useful for wafer testing, which employs structures and materials which facilitate repeatable probe manufacture without resort to nonstandard methods or materials.
Lastly, but not limited hereto, it is an object of the present invention to provide probe wires and a probe assembly which is capable of making firm and consistently uniform connections to pads on semiconductor wafers during their testing.
The recitation herein of a list of desirable objects which are met by various embodiments of the present invention is not meant to imply or suggest that any or all of these objects are present as essential features, either individually or collectively, in the most general embodiment of the present invention or in any of its more specific embodiments.